Noise limiter



Dec. 22, 1942. Q Q MARTlNELL. 2,305,931

NOISE LIMITER Filed Aug. 21, 1941 IQTDET LI.) 2 1 5 E GATE Q g VOLTAGE uVOLTS p/CTQRE VOLTAGE, V v Zhmentor I ,vous Cum C. MARTmELu I BLACKNOISE I WHITE Gttomeg Patented Dec. 22, 1942 NOISE LDIITER Ciro o.Martinelli, Oaklyn, N. 3., assignor to Radio Corporation of America, acorporation of Delaware Application August 21, 1941, Serial No. 407,739

5 Claims.

My invention relates to noise limiters for radio receivers and,particularly, to limiters applied to the picture channel of a televisionreceiver.

In my copending application Serial No. 324,239, filed March 16, 1940,and entitled "Noise limiters, there is described and claimed a preferrednoise limiter for use in a picture channel, this limiter having a fixedlimiter or gate voltage and being direct current connected in thepicture channel. With a limiter circuit of this character, the noiseclipping level remains fixed regardless of signal strength with theresult that, for the weaker incoming signals, the clipping level issubstantially beyond a 100% modulated signal. As a result, a substantialpart of the noise is not removed.

An object of the present invention is to provide an improved noiselimiter, especially adapted for use in the picture channel of atelevision receiver.

A further object of the invention is to provide an improved noiselimiter for the picture channel of a television receiver, the limiterbeing supplied with a limiter or gate voltage which varies withvariations in the strength of the incoming signal.

A still further object of the invention is to provide an improved noiselimiter for a picture signal channel in a television system of the typeemploying direct current transmission.

In one of the preferred embodiments of the invention *as applied to atelevision receiver, a series type noise limiter is coupled to thesecond detector of the picture channel'and supplied with a gate voltagewhich is obtained from a peak rectifier coupled to the second detectorinput circuit. The connection is such that the gate voltage depends uponthe peak value of the synchronizing pulses and, therefore, changesinresponse to changes in the signal strength such as those caused byfading or by tuning the receiver to a different television transmitter.

Where the limiter is employed in a picture channeL it is preferred thatthe picture signal be applied to the limiter through a direct currentconnection in order that the noise may be clipped off at substantially100% modulation regardless of changes in the picture background.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing in which Figure l is acircuit diagram showing one embodiment of the invention applied to atelevision receiver,

Figures 1a and 1b are curves which illustrate the operation of the noiselimiter shown in Fig. 1,

Figure 2 is a circuit diagram illustrating one of the Preferredembodiments of the invention as applied to a television receiver,

Figure 2a is a groupof curves illustrating the operation of the noiselimiter shown in Fig. 2.

In the several figures, like parts are designated by the same referencecharacters.

Referring to Figure 1, the invention is shown applied to a televisionreceiver comprising a first detector and tunable oscillator |0, an I.-F.amplifier II and a second detector I2. The detector '|2 may be coupledto the amplifier through a coupling circuit comprising a tunable primarycoil I4 connected to a point on a secondary coil I6 through a couplingcondenser II.

In series with the detector diode I2, there is a load resistor |8 whichis shunted by a condenser 9 for bypassing the I.-F. carrier wave.

The video signal appears across resistor I8 and is supplied to the noiselimiter circuit which comprises a diode 2| having an anode 26 connectedthrough a conductor 20 to the plate of the second detector l2 and havinga cathode 22 com nected to ground through a signal output resistor 26.

The noise limiter operates on the same principle as that described in myabove-identified copending application, namely, alimiter or gate voltageis applied to the diode 2| with the proper polarity to cause a flow ofdiode current while the signal voltage is applied to the limiter circuitwith the opposite polarity. Thus, the flow of diode current ceases iithe amplitude of noise in the signal voltage exceeds the amplitude ofthe gate voltage and there is no longer a flow of current through theoutput resistor 26.

In accordance with a preferred embodiment of my invention, the gatevoltage is obtained by means of a peak rectifier circuit comprising adiode 2'Ihaving in series therewith an RC circuit consisting of aresistor 28 and a condenser '29, the diode 21 and the RC circuit havingI.-F. signal applied thereto from a coil 3| which is close coupled tothe secondary coil IS.

The time constant of the RC circuit '28-'29 is such that the voltageproduced thereacross is a D. C. voltage which changes only with changesin signal strength. This is the gate voltage which is applied to thelimiter diode 2| through a circuit which may be traced-from the cathodeend of resistor 28, through a resistor 23 and a conductor 32 to theanode 240i the limiter diode 2|, through the limiter diode and outputresistor 25 to ground, and through ground to the other end or resistor28. The resistor 23 is provided to prevent condenser 29 from bypassingresistor l8.

The second detector output is applied across the series combination oflimiter diode 2| and output resistor by means of the conductor 20 and acondenser 33 connected between the lower end of' resistor I8 and ground,the polarity of the applied signal being that the noise pulses (in thedirection of black in the picture) oppose current flow in the diode.

The operation of the above-described limiter circuit is illustrated inFigures 1 and If). It is assumed that the incoming signal for thepicture channel is a negatively modulated carrier wave and that themodulating signal is the usual mixture of picture signal, including thepicture D. C. component, and synchronizing signal such as defined by'the present R. M. A. standards. The latter signal includes horizontalsynchronizing pulses occurring at the end of each scanning line and aslotted vertical synchronizing pulse occurring at the end of eachvertical deflection. In Figs. 1a and 1b, the horizontal synchronizingpulses are indicated at 36 and 36', the pulses 36 occurring during adark picture, and the pulses 36' occurring during a light picture.

The curves of Figs. 1a and 1b represent conditions for a strong incomingsignal and for a weak incoming signal, respectively. A comparison of thetwo figures shows that the limiter or gate voltage appearing acrossresistor 28 (Fig. 1) is greater for the strongsignal (Fig. 1a) than forthe weak signal (Fig. lb). Therefore, more noise is clipped off the Weaksignal than would be the case if the gate voltage remained at the valuerequired for the stronger signal.

It will be noted that, since the signal is applied to the limiter inFig. 1 through an A. C. connection (the condenser 33 breaking the D. C.path), the noise is not clipped off as close to the synchronizing pulsesfor a dark or black picture as fora light or white one. Thisdisadvantage may be avoided by employing the circuit shown in Fig. 2.

Referring to Fig, 2, the gate voltage which appears across resistor 28and the signal voltage which appears across resistor I8 are applied inseries relation to the limiter diode 2! rather than in parallel relationas in Fig. 1. This is done in the example illustrated by connecting thelower end of resistor I8 to the cathode end of resistor 28 through aconductor 4|, instead of to ground through a condenser as in the firstcircuit. It will be seen that the signal output of the second detectoris now applied to the limiter through a D. C. coupling.

The advantage of the circuit of Fig. 2 is illustrated in Fig. 2a. Sincethe D. C. component of the picture signal is not lost at the-limiter,the synchronizing pulses for pictures having a dark background willremain at the same level as synchronizing pulses for pictures having alight background. Consequently,'thesame percentage of the noise isremoved regardless .of the background component of the transmittedpicture. It will be understood, also, that the gate voltage varies withchanges in signal strength, just as described in connection with Fig. l,to insure removal of most of the noise for weaker signals as well as forstrong signals.

I claim as my invention:

1. A television receiver for the reception of a carrier wave negativelymodulated by a composite signal consisting of picture signals andsynchronizing pulses, said receiver comprising means for demodulatingsaid received signal to produce picture signals and synchronizingpulses, a signal amplitude limiter which is connected to receive signalfrom said demodulating means and which comprises a diode and an outputresistor connected in series, and peak rectifier means for inserting adirect current voltage in series with said series combination of diodeand output resistor with the proper polarity to maintain a fiow ofcurrent through said diode until the signal to be limited exceeds apredetermined amplitude and with an amplitude that increases in responseto an increase in signal strength.

2. A television receiver for the reception of a carrier wave negativelymodulated by a composite signal consisting of picture signals andsynchronizing pulses and also modulated by a direct current component ofthe transmitted picture, said receiver comprising means for demodulatingsaid received signal to produce picture signals and synchronizingpulses, a signal ampli tude limiter which is'direc't current connectedto said demodulating means and which comprises a diode and an outputresist'or connected in se-. ries, and peak rectifier means forinserting-a direct current voltage in series with said seriescombination of diode and-output resistor with the proper polarity tomaintain a flow ofcurrent through said diode until the signal to belimited exceeds a predetermined amplitude and with an amplitude thatincreases in response to an in crease in signal strength.

3. In a television system of the type in which the direct currentcomponent of the picture is transmitted whereby the carrier wave variesin accordance with the changes in background of the transmitted pictureand in which the carrier wave is negatively modulated by the picturesignal and by synchronizing pulses, a receiver comprising means fordemodulating said transmitted picture signals, and a signal amplitudelimiter through which the output of said demodulatin means is led to autilization circuit, said limiter being direct current connected to saiddemodulating means and comprising a diode and an output resistorconnected in series, means for inserting a direct current voltage inseries with said series combinationof diode and output :6- sistor withthe proper polarity to maintain a flow of current through said diodeuntil the signal to be limited exceeds a predetermined amplitude andwith an amplitude which varies in accordance with the peak amplitude ofsaid synchronizing pulses, and means for applying said signal across theseries combination of diode, output resistor and D. C. voltage with suchpolarity as to oppose current flow through said diode.

4. A television receiver for the reception of a carrier wave negativelymodulated by a composite signal consisting of picture signals andsynchronizing pulses and also modulated by the direct current componentof the transmitted picture, said receiver comprising means fordemodulating said received signal to produce picture signals andsynchronizing pulses, said demodulating means including an outputresistor, and an amplitude limiter circuit comprising a diode, an outputresistor and peak rectifier means for providing a direct current voltagein series with said diode and said output resistor which increases inamplitude inresponse to an increase in the peak amplitude of saidpulses, said series combination of diode, output resistor and directcurrent means being direct current connected across the output resistorof said demodulating ite signal consisting of picture signals andsynchronizing pulses and also modulated by the direct current componentof the transmitted picture, said receiver comprising means fordemodulating said received signal to produce-picture signals andsynchronizing pulses, said demodulating means including an outputresistor, and an amplitude limiter circuit comprising a diode,

an output resistor andpeak rectifier. means for providing a directcurrent voltage in series with said diode and said output resistor whichin.- creases in amplitude in response to an increase in the peakamplitude of said pulses, said series combination, of diode, outputresistor and peak rectifier being direct current connected in series fedrelation to the output resistor of said demodulating means, said diodebeing connected in such a direction in the circuit that-thesynchronizing pulses appearing across the demodulator output resistoroppose current flowthrough the diode, and said direct current voltagehaving the proper polarity to maintain a flow of current through thediode so long as the signal appearing across the demodulator outputresistor having the polarity of the synchronizing pulses does not exceeda predetermined amplitude.

CD30 0. MARTINELH.

